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Infrasonic Detection of Tornadoes and Tornadic Storms Summary There Is Infrasonic Detection of Tornadoes and Tornadic Storms Summary There is evidence that infrasonic observing systems have significant potential to improve tornado warnings. A key section 8 in this background paper summarizes six case studies, describing infrasonic detection of tornadoes, tornadic storms, and funnels under various conditions and summarizing warning times. The near infrasound systems applied in this research provide a new way of “hearing” low- frequency sounds in the atmosphere and focus on a different, higher frequency window than historical infrasonic research dating to the 1970’s. The purposes of this paper are to provide background and a status report on this tool for tornado detection. This technology is at an ideal point for demonstration in an operational context. 1 Table of Contents by Section 1. What is infrasound? P3 2. What sensors and techniques are required for the P4 detection of these low-level, low frequency sounds? 3. At what distances from a source can infrasound P6 be detected? 4. But hasn’t the use of infrasound for monitoring P6 severe weather been researched since the 1970’s with no operational uses resulting? 5. A Typical Infrasonic Summer Day P7 6. A contrast to the quiet summer day P11 7. How can vortices generate sound? P13 8. Case studies of infrasound associated with tornadoes P14 and tornadic storms 9. But are there regional signals that can cause false alarms? P19 10. How can Infrasonic Observatories Improve P20 Tornado Warnings? 11. What are the next steps? P21 12. What form will the prototype network take? P21 –and- What steps will be critical to assessment? 13. What are the Anticipated Benefits-to-Cost Ratios? P24 14. References P25 2 Infrasonic Detection of Tornadoes 1. What is infrasound? Infrasound is the range of acoustic frequencies below the audible (Bedard and Georges, 2000). For a typical person this is at frequencies below about 20 Hz, where the threshold of human hearing and feeling crossover. There is a rational analog between sound and the relationship of infrared to visible light. Thus, one can call the frequency range 1 to 20 Hz near infrasound and the range from about .05 to 1 Hz infrasound. Below about .05 Hz where gravity becomes important for propagation, atmospheric waves are usually called acoustic/gravity waves. Figure 1 indicates the sound pressure levels as a function of frequency with the threshold of human hearing as a reference. A frequency of 1 Hz is eight octaves below middle C. The lowest frequency on a piano keyboard is A at 27.5 Hz, consistent with being near the lowest limits of a typical persons hearing. Acousticians have adopted the convention of defining sound levels relative to the threshold of human hearing, and most situations involve the perceptions of people to sound. However, the infrasonic signal data can be quite valuable and provide information on a range of geophysical processes. More background on infrasonics may be found on the web site http://www.etl.noaa/et1/infrasound/. 140 -102 THRESHOLD OF PAIN 120 -101 JET ENGINE 100 ROCK MUSIC -100 INFRASOUND VACUUM CLEANER 80 -1 T -10 H R ESH Pa 60 OL SPEECH -2 GEOPHYSICAL SOURCES D -10 O Sound Pressure, dB F H EAR 40 SURF IN -3 G -10 TYPICAL LIMIT OF PRESSURE 20 SENSOR SENSITIVITY -10-4 0 0.11 10 100 1000 Frequency, Hz Fig 1 Sound pressure amplitude as a function of frequency compared with the threshold of human hearing at lower frequencies 3 2. What sensors and techniques are required for the detection of these low-level, low frequency sounds? A critical need was the development of an effective method for reducing noise from large but highly spatially incoherent pressure fluctuations, while still detecting infrasonic signals. Daniels (1959) made the critical breakthrough in devising a noise reducing line microphone. His concept was to match the impedance along a pneumatic transmission line using distributed input ports and pipe size changes to minimize attenuation. This innovation exploiting the high spatial coherence of infrasound and the small spatial coherence of pressure changes related to turbulence, provided signal-to-noise ratio improvements of the order of 20 dB. Variations of his concept are currently in use worldwide. Figure 2 is a photo of a spatial filter using 12 radial arms with ports at one-foot intervals and covering a diameter of fifty feet. We have adapted porous irrigation hose for use as a distributed pressure signal transmission line (an infrasonic noise reducer), saving considerable costs in fabrication and maintenance. The present wind noise reducers can look a lot like an octopus (with four extra arms) with black porous hoses radiating outward from a central sensor. Figure 2. Photograph of a spatial filter for reducing wind-induced pressure fluctuations in the infrasonic frequency range. A reasonable assumption, for sound waves from point sources after traveling paths of tens or hundreds of wavelengths, is that the wave fronts are planar. This model is usually used in processing data from infrasonic observing systems and was the basis for the design of beamsteering algorithms developed to determine correlation coefficient, azimuth, and horizontal 4 phase speed. Effective separations for microphones in arrays are usually about ¼ of the primary acoustic wavelength to be detected. In addition, the fact that infrasonic signals show little change with distance is the basis of the method for the reduction of unwanted pressure noise. Figure 3 shows a typical array layout. Usually an infrasonic observatory consists of 4 sensors equipped with spatial filters in a rough square configuration. The twelve porous hoses are often 50-feet in length and bent back toward the center so that the complete filter covers a diameter of about 50 feet. These noise reducers can operate in rain and snow without affecting infrasonic signals. Vegetation can reduce wind in the lower boundary layer and further improve wind noise reduction. Array Geometry 25 Feet 25 Feet - 25 Feet 25 Feet 40 to 80 meter typical spacings Figure 3. View of a typical infrasonic observatory array configuration. The exact positioning of the twelve porous irrigation hoses radiating outward from each of the four sensors is not critical. Conventional microphones being designed for audio frequencies do not respond to infrasonic frequencies. There are a number of reasons for this, in addition to the fact that there was no practical need to detect inaudible sounds. For example, extended low frequency sensitivity could cause undesirable dynamic response limitations. Thus, a microphone having a diaphragm deflecting in response to sound waves typically has a leakage path to a reference volume behind the membrane. This permits frequencies below the cutoff of such a high pass filter to appear on both sides of the sensor, canceling response. We use sensitive differential pressure sensors integrated with a large, well-defined reference volume and calibrated flow resistor providing a stable high-pass filter time constant. The volume is insulated to create a stable temperature environment. These sensors are rugged, relatively small (less than 2x2x2 feet), and relatively light (about 22 pounds). We typically operate for years with few problems. Because arrays of 5 sensors are applied to detect and process signals, there is a need to match the microphone sensitivities and phase characteristics. This required the creation of a family of static and dynamic pressure calibration techniques, as well as methods for measuring flow resistance. The infrasonic sensors are carefully matched and interchangeable. 3. At what distances from a source can infrasound be detected? Cook (1962) has shown how unimportant molecular attenuation is to infrasonic propagation ( 5x10-8 dB per km ), whereas sound at 2 kHz will attenuate at 5 dB per km. Since infrasound below 1Hz is virtually unattenuated by atmospheric absorption, it is detectable at distances of thousands of kilometers from the source. Thus, an infrasonic wave at a frequency lower than 1Hz will travel for 1000 kilometers with less attenuation than a sound wave at 2 kHz traveling for 1000 feet. Temperature changes in the atmosphere affect infrasound in the same way that light waves are refracted by lenses. In addition, wind speed changes contribute to sound refraction, guiding sound waves as they travel for long distances. Some waves escape and travel upwards to great heights in the ionosphere where they dissipate, while other sound rays are trapped and bounce back and forth many times between the earth’s surface and the upper atmosphere as they travel horizontally. The atmosphere acts as a waveguide trapping much of the acoustic energy Georges and Beasley (1977). 4. But hasn’t the use of infrasound for monitoring severe weather been researched since the 1970’s with no operational uses resulting? Most past observations used acoustic passbands much lower in frequency than the 0.5- to 5-Hz frequency range currently focused on. There are significant differences between the infrasonic measurement systems applied in the 1970’s and 1980’s as a part of a global observing network and the higher frequency near infrasound systems now in use. Several features of the global observing system (including the array dimensions, the spatial filter, and the sensor itself) combined to limit the high frequency response to below 0.5 Hz. The severe weather-related infrasound reported in the literature from the historic global infrasonic network involves sound two orders in magnitude longer in wavelength than the newer system and at continental scales of 1000’s of kilometers (e.g. Bowman and Bedard, 1971, Georges and Greene, 1975). In contrast, the near infrasound system used to take the current measurements outlined here focuses on frequencies in the range .5 to 10 Hz and regional range scales of 100’s of kilometers or less.
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